1. Field of the Invention
One embodiment of the present invention relates to light-emitting elements having a plurality of emission colors. One embodiment of the present invention also relates to light-emitting devices, electronic devices, and lighting devices. One embodiment of the present invention further relates to novel pyrene-based compounds.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. In addition, it is also a great advantage that the light-emitting element can be manufactured as a thin and lightweight element. Furthermore, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in the form of a film, they make it possible to provide planar light emission easily. Therefore, large-area elements using planar light emission can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources which can be applied to lighting devices and the like.
Light-emitting elements utilizing electroluminescence are broadly classified according to whether they include an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as a light-emitting substance, application of voltage to a light-emitting element causes electrons and holes to be injected into a layer containing the light-emitting organic compound from a pair of electrodes, whereby current flows. The carriers (electrons and holes) are recombined, and thus the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Note that the excited state of an organic compound can be a singlet excited state and a triplet excited state, and light emission from the singlet excited state (S*) is referred to as fluorescence, and light emission from the triplet excited state (T*) is referred to as phosphorescence.
A function as a light-emitting layer of a light-emitting element formed using a light-emitting organic compound can be achieved with the light-emitting organic compound alone. However, a method for forming a light-emitting layer in which a light-emitting organic compound is dispersed in a matrix of another substance is also employed for the purpose of preventing concentration quenching of the light-emitting organic compound, for example. Note that a substance serving as a matrix is called a host material, and a substance dispersed in the matrix is called a guest material.
In that case, carriers (electrons and holes) injected from both electrodes are recombined in the host material of the light-emitting layer, and the guest material receives the energy and emits light. Therefore, it is known that light emission with high luminance and high color purity can be achieved.
In addition, light-emitting elements which emit white light have recently been employed for lighting purposes. In such a case, white light emission can be achieved with the use of a plurality of light-emitting materials. However, a light-emitting layer containing plural kinds of light-emitting materials causes problems such as a change in chromaticity and a decrease in external quantum efficiency, due to energy transfer between the light-emitting materials.
Against these problems, it has been proposed to stack a plurality of light-emitting layers, each containing a different light-emitting material (see, for example, Reference 1). However, in that case, a problem is a high possibility of deterioration because of charge accumulation at the interface between the stacked layers, for example.